The present invention relates to a device for the polarographic measurement of oxygen pressure, and in particular to such a device wherein the measuring head of the device is to be brought into mechanical contact with the object to be measured.
It is already known to provide a device of the general character with which the present invention is concerned, but a device which cannot be calibrated. This prior-art device may for instance utilize a plurality of platinum wires (Pflugers Arch, 337,185- 198/1972) which are distributed over a surface area of approximately 0.5-1 cm.sup.2 and are melted into a glass body acting as a carrier. There is further provided an Ag/AgCl reference electrode which is adjacent to the platinum wires, for instance surrounding them in form of an annulus. The arrangement is in accordance with the teachings of Clark Trans. Amer. Soc. Art. Int. Org. 2,41(1956) and Luebbers Pflugers Arch 271,431/1960 and is covered with a Teflon (TM) foil which should be as thin as possible and with a cuprophane (TM) foil, which is an especially thin cellophane type foil.
It has been found that this type of device will produce reproduceable measuring results if it is carefully placed upon an object to be measured, for instance the surface of an organ of the human body. This was proven with the electrode suggested by R. Huch and disclosed on page 7 of her structural thesis delivered in 1971 in Marburg, Germany.
It is known that the calibration of oxygen electrodes requires that the measuring surface of the device is subjected to gaseous oxygen of known pressure, and that the oxygen reduction current is then measured. However, the thus obtained calibration results cannot be readily utilized for measuring the oxygen pressure in other media, for instance on the skin of a human being, because, though in this instance the reduction current will always be proportional to the oxygen quantity which reaches the measuring electrode, the relationship between reduction current and oxygen pressure is unknown because the oxygen reaches the measuring electrode only by diffusion through an unknown medium. Therefore it is necessary to keep in mind, in this context, that for purposes of calibrating the device it is the diffusion characteristics of the device alone which are of predominant importance, whereas under actual measuring conditions it is necessary to take into account both the diffusion characteristics of the device and the diffusion characteristics of the medium (i.e., object) to be measured. The term "diffusion" herein refers, of course, to the diffusion of the oxygen through the device to the measuring electrode of the latter, and through the medium whose oxygen pressure is to be measured.
Because of this second consideration, however, namely the fact that under actual measuring conditions it is necessary to also take into account the diffusion characteristic of the medium to be measured, the calibration results obtained during the previous calibration of the device cannot be used without difficulties for measuring the oxygen pressure in an actual medium, and this causes considerable difficulties in actual practice.